I think it depends on what the reason is that's causing you to not play well that day. For example, eye strain can cause a player to have a bad day, or being weak from not eating enough that day, or being too full and lethargic after a big meal, or not getting enough sleep where it finally catches up to you.
The key is to pick up on potential issues before you decide to play that day. In more cases than you would think, there is usually an underlying issue not related to pool that causes an off-day. So, should you play through it? It depends on the issue. For example if your eyes are tired and strained, you should rest them. If you had a big meal, your body is putting a lot of energy into digesting the food. It will take some time before it wears off.
Tension is another game-killer. If you walk into a pool room tense, it will take some time before you can relax and start to play well. Doing a short physical exercise routine before leaving for the pool room helps a lot.